In recent years, touch sensor panels, touch screens, and the like have become available as input devices. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device, such as an LCD panel, that can be positioned partially or fully behind the touch sensor panel or integrated with the touch sensor panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching (or nearly touching) the touch sensor panel using one or more fingers, styli or other objects at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and a computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
The touch sensor panel can be constructed as an array of touch electrodes and/or pixels, each electrode or pixel capable of sensing a touch event (one or more touches or near-touches) occurring at the location of the electrode or pixel. To determine the locations of one or more touch events, a scan of the touch sensor panel can be performed during which time one or more pixels or electrodes can be evaluated in sequence for touch event activity until an “image” of touch is obtained for the entire panel.
However, depending on the design of the touch sensor panel, the electrodes of a particular panel may have different intrinsic capacitances due, for example, to being different in size and shape and being connected to a touch controller with routing traces of different lengths (i.e., the series resistance of the traces may vary based on their length). As a result, touch sensitivity of the touch panel sensor may not be uniform everywhere on the panel. In addition, changes in temperature and/or humidity can also affect the sensitivity of the touch sensor panel. Without proper calibration, touch sensor panels may not always perform as expected in some conditions.